tailscale/wgengine/filter/filter.go

// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package filter contains a stateful packet filter.
package filter

import (
	"sync"
	"time"

	"github.com/golang/groupcache/lru"
	"golang.org/x/time/rate"
	"tailscale.com/types/logger"
	"tailscale.com/wgengine/packet"
)

type filterState struct {
	mu  sync.Mutex
	lru *lru.Cache // of tuple
}

// Filter is a stateful packet filter.
type Filter struct {
	logf logger.Logf
	// localNets is the list of IP prefixes that we know to be "local"
	// to this node. All packets coming in over tailscale must have a
	// destination within localNets, regardless of the policy filter
	// below. A nil localNets rejects all incoming traffic.
	localNets []Net
	// matches is a list of match->action rules applied to all packets
	// arriving over tailscale tunnels. Matches are checked in order,
	// and processing stops at the first matching rule. The default
	// policy if no rules match is to drop the packet.
	matches Matches
	// state is the connection tracking state attached to this
	// filter. It is used to allow incoming traffic that is a response
	// to an outbound connection that this node made, even if those
	// incoming packets don't get accepted by matches above.
	state *filterState
}

// Response is a verdict: either a Drop, Accept, or noVerdict skip to
// continue processing.
type Response int

const (
	Drop Response = iota
	Accept
	noVerdict // Returned from subfilters to continue processing.
)

func (r Response) String() string {
	switch r {
	case Drop:
		return "Drop"
	case Accept:
		return "Accept"
	case noVerdict:
		return "noVerdict"
	default:
		return "???"
	}
}

// RunFlags controls the filter's debug log verbosity at runtime.
type RunFlags int

const (
	LogDrops RunFlags = 1 << iota
	LogAccepts
	HexdumpDrops
	HexdumpAccepts
)

type tuple struct {
	SrcIP   packet.IP
	DstIP   packet.IP
	SrcPort uint16
	DstPort uint16
}

const lruMax = 512 // max entries in UDP LRU cache

// MatchAllowAll matches all packets.
var MatchAllowAll = Matches{
	Match{[]NetPortRange{NetPortRangeAny}, []Net{NetAny}},
}

// NewAllowAll returns a packet filter that accepts everything to and
// from localNets.
func NewAllowAll(localNets []Net, logf logger.Logf) *Filter {
	return New(MatchAllowAll, localNets, nil, logf)
}

// NewAllowNone returns a packet filter that rejects everything.
func NewAllowNone(logf logger.Logf) *Filter {
	return New(nil, nil, nil, logf)
}

// New creates a new packet filter. The filter enforces that incoming
// packets must be destined to an IP in localNets, and must be allowed
// by matches. If shareStateWith is non-nil, the returned filter
// shares state with the previous one, to enable rules to be changed
// at runtime without breaking existing flows.
func New(matches Matches, localNets []Net, shareStateWith *Filter, logf logger.Logf) *Filter {
	var state *filterState
	if shareStateWith != nil {
		state = shareStateWith.state
	} else {
		state = &filterState{
			lru: lru.New(lruMax),
		}
	}
	f := &Filter{
		logf:      logf,
		matches:   matches,
		localNets: localNets,
		state:     state,
	}
	return f
}

func maybeHexdump(flag RunFlags, b []byte) string {
	if flag == 0 {
		return ""
	}
	return packet.Hexdump(b) + "\n"
}

// TODO(apenwarr): use a bigger bucket for specifically TCP SYN accept logging?
//   Logging is a quick way to record every newly opened TCP connection, but
//   we have to be cautious about flooding the logs vs letting people use
//   flood protection to hide their traffic. We could use a rate limiter in
//   the actual *filter* for SYN accepts, perhaps.
var acceptBucket = rate.NewLimiter(rate.Every(10*time.Second), 3)
var dropBucket = rate.NewLimiter(rate.Every(5*time.Second), 10)

func (f *Filter) logRateLimit(runflags RunFlags, q *packet.ParsedPacket, r Response, why string) {
	var verdict string

	if r == Drop && (runflags&LogDrops) != 0 && dropBucket.Allow() {
		verdict = "Drop"
		runflags &= HexdumpDrops
	} else if r == Accept && (runflags&LogAccepts) != 0 && acceptBucket.Allow() {
		verdict = "Accept"
		runflags &= HexdumpAccepts
	}

	// Note: it is crucial that q.String() be called only if {accept,drop}Bucket.Allow() passes,
	// since it causes an allocation.
	if verdict != "" {
		b := q.Buffer()
		f.logf("%s: %s %d %s\n%s", verdict, q.String(), len(b), why, maybeHexdump(runflags, b))
	}
}

// RunIn determines whether this node is allowed to receive q from a Tailscale peer.
func (f *Filter) RunIn(q *packet.ParsedPacket, rf RunFlags) Response {
	r := f.pre(q, rf)
	if r == Accept || r == Drop {
		// already logged
		return r
	}

	r, why := f.runIn(q)
	f.logRateLimit(rf, q, r, why)
	return r
}

// RunOut determines whether this node is allowed to send q to a Tailscale peer.
func (f *Filter) RunOut(q *packet.ParsedPacket, rf RunFlags) Response {
	r := f.pre(q, rf)
	if r == Drop || r == Accept {
		// already logged
		return r
	}
	r, why := f.runOut(q)
	f.logRateLimit(rf, q, r, why)
	return r
}

func (f *Filter) runIn(q *packet.ParsedPacket) (r Response, why string) {
	// A compromised peer could try to send us packets for
	// destinations we didn't explicitly advertise. This check is to
	// prevent that.
	if !ipInList(q.DstIP, f.localNets) {
		return Drop, "destination not allowed"
	}

	switch q.IPProto {
	case packet.ICMP:
		if q.IsEchoResponse() || q.IsError() {
			// ICMP responses are allowed.
			// TODO(apenwarr): consider using conntrack state.
			//  We could choose to reject all packets that aren't
			//  related to an existing ICMP-Echo, TCP, or UDP
			//  session.
			return Accept, "icmp response ok"
		} else if matchIPWithoutPorts(f.matches, q) {
			// If any port is open to an IP, allow ICMP to it.
			return Accept, "icmp ok"
		}
	case packet.TCP:
		// For TCP, we want to allow *outgoing* connections,
		// which means we want to allow return packets on those
		// connections. To make this restriction work, we need to
		// allow non-SYN packets (continuation of an existing session)
		// to arrive. This should be okay since a new incoming session
		// can't be initiated without first sending a SYN.
		// It happens to also be much faster.
		// TODO(apenwarr): Skip the rest of decoding in this path?
		if q.IPProto == packet.TCP && !q.IsTCPSyn() {
			return Accept, "tcp non-syn"
		}
		if matchIPPorts(f.matches, q) {
			return Accept, "tcp ok"
		}
	case packet.UDP:
		t := tuple{q.SrcIP, q.DstIP, q.SrcPort, q.DstPort}

		f.state.mu.Lock()
		_, ok := f.state.lru.Get(t)
		f.state.mu.Unlock()

		if ok {
			return Accept, "udp cached"
		}
		if matchIPPorts(f.matches, q) {
			return Accept, "udp ok"
		}
	default:
		return Drop, "Unknown proto"
	}
	return Drop, "no rules matched"
}

func (f *Filter) runOut(q *packet.ParsedPacket) (r Response, why string) {
	if q.IPProto == packet.UDP {
		t := tuple{q.DstIP, q.SrcIP, q.DstPort, q.SrcPort}
		var ti interface{} = t // allocate once, rather than twice inside mutex

		f.state.mu.Lock()
		f.state.lru.Add(ti, ti)
		f.state.mu.Unlock()
	}
	return Accept, "ok out"
}

func (f *Filter) pre(q *packet.ParsedPacket, rf RunFlags) Response {
	if len(q.Buffer()) == 0 {
		// wireguard keepalive packet, always permit.
		return Accept
	}
	if len(q.Buffer()) < 20 {
		f.logRateLimit(rf, q, Drop, "too short")
		return Drop
	}

	switch q.IPProto {
	case packet.Unknown:
		// Unknown packets are dangerous; always drop them.
		f.logRateLimit(rf, q, Drop, "unknown")
		return Drop
	case packet.IPv6:
		f.logRateLimit(rf, q, Drop, "ipv6")
		return Drop
	case packet.Fragment:
		// Fragments after the first always need to be passed through.
		// Very small fragments are considered Junk by ParsedPacket.
		f.logRateLimit(rf, q, Accept, "fragment")
		return Accept
	}

	return noVerdict
}